The present invention relates to the processing of a C3 to C6 hydrocarbon cut, such as from a steam cracking process or a fluid catalytic cracking process, primarily for conversion of C4 and C5 olefins to propylene via metathesis.
In typical olefin plants, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene, and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C3 to C6. This mixture is separated into different carbon numbers typically by fractionation.
The C3 cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C3 compounds end up as propylene thereby increasing the yield.
The C4 cut consisting of C4 acetylenes, butadiene, iso and normal butenes, and iso and normal butane can be processed in many ways. A typical steam cracker C4 cut contains:
The C4 acetylenes are first removed by selective hydrogenation followed by butadiene extraction. Alternately they are hydrogenated along with butadiene to form butenes. Isobutene can be removed by fractionation, by reaction to methyl tertiary butyl ethylene using methanol, or by reaction with itself and normal butenes in a catalytic C4 dimerization unit. The isobutene is typically removed before metathesis with ethylene since it is known to cause catalyst fouling when present in both concentrations greater than 10%. The C5 and heavier stream is typically used in the production of gasoline but sometimes the C5""s are separated and recycled to the cracking heaters.
The bottoms containing primarily the 1-butene and 2-butene are mixed with ethylene and passed through the metathesis or olefin conversion reacting step. In this conversion reaction step, the primary reaction is:
2-butene+ethylenexe2x86x92(2)propylene
The catalyst for this reaction is typically an oxide of Group VI A or Group VII A metals supported on either alumina or silica supports. In some cases, this oxide is admixed with a double bond isomerization catalyst such as MgO. In the reactor, the 2-butene and ethylene are metathesised to propylene. The 1-butene does not react with ethylene. The isomerization catalytic activity incorporated allows 1-butene to be isomerized to 2-butene which is then reacted with the ethylene. The effluent containing propylene, unreacted ethylene and butenes and some C5 and heavier products is first passed through a deethylenizer for removal of that unreacted ethylene and then to a depropylenizer where product propylene is removed overhead. The bottoms may be sent to a debutanizer where unreacted C4s are recovered and recycled. The C5 and heavier fraction is typically sent to gasoline. Alternately, a C4 stream is withdrawn from the depropyleneizer above the bottoms and recycled with the net bottoms of C5 and heavier again being sent to gasoline.
In the conventional process such as generally described above, there are several problems or disadvantages. First, there are separate fixed bed hydrogenation units for the C3""s and butadiene. In the butadiene hydrogenation step, if high 2-butene concentrations are desired, additional hydrogenation is specified in order to maximize the hydroisomerization of 1-butene to 2-butene. In the hydroisomerization of 1-butene to 2-butene in the selective butadiene hydrogenation unit, there is a substantial loss (10+%) of butenes to paraffins which represents a considerable feed loss to the conversion step. Further, if fractionation is employed for the isobutene removal step, there is an additional loss of butenes since 1-butene is difficult to separate from isobutene without a very expensive fractionation tower. There typically is a 60% conversion of C4""s to propylene in the metathesis step. If there is a side draw of C4""s from the depropylenizer for C4 recycle, there can be a buildup of C5 and heavier compounds due to the limitations on purity which needlessly load the conversion reactor. In addition, the C4""s that are recycled have a high concentration of 1-butene due to the isomerization reaction within the metathesis bed.
The object of the present invention is to provide an improved process for the conversion of olefins to maximize the production of propylene from a C3 to C6 cut from a steam or other cracking process. The invention involves the replacement of some separate hydrogenation steps with catalytic distillation hydrogenation operating along with an olefin conversion reaction to provide the maximum isomerization of 1-butene to 2-butene and the maximum metathesis of 2-butene and ethylene to propylene.